30 



AXXALS XEW YORK ACADEMY OF SCIEXCES 



andesitic makeup. The present condition of these rocks, representing as 

 they do nearly all stages between fresh material and either a thoronghlY 

 weathered or considerably metamorphosed condition, is a more interest- 

 ing stndy than their primary composition. Some of the most dense and 

 resistant rocks in the whole island are these older metamorphosed tuffs 

 and ashes. 



N'ext in point of abundance is the group of crystalline igneous rocks. 

 In this case there is somewhat greater prominence of varieties represent- 



FlG. 



-Photomicrograph of a tupical thoroughly indurated andesitic ash, magnification 



2S diameters 



A rock of this type appears in the field as a dark-greenish hard resistant obscurely 

 bedded layer, usually closely associated with more massive tuffs on the one hand or 

 more strongly bedded shales on the other. The clear grains are mineral fragments ; the 

 more complex grains are fragments of lava, cinders, glass, etc., all thoroughly bound 

 into a complex aggregate. 



ing the acid and basic ends of the classification scheme, but here also the 

 rocks of the andesite-diorite family are by all means the most numerous 

 and most widely distributed. The greater number of occurrences are 

 represented by members of this family l^elonging to intrusives that would 

 be classed normally as andesite porphyriest porphyrites of various kinds 

 and diorite porphyries. The minor variations represented by these rocks 



